1. Field of the Invention
The present invention relates to a radiographic imaging apparatus such as an X-ray computed tomography (CT) apparatus that images a radiation attenuation distribution of a subject acquired using radiated rays such as X-rays. More particularly, the present invention relates to a CT apparatus using cone beams in which a relationship between a fan angle and a heart beat period is determined so that occurrence of artifacts caused by the heart beat is reduced.
2. Description of the Related Art
Known X-ray CT apparatuses emit X-rays onto subjects and X-ray detectors detect the X-rays that have passed through the subjects or have been scattered off the subjects, and radioscopic images, cross-sectional images, or three-dimensional images of the subjects are acquired based on the outputs of the detected X-rays (the number of photons of the X-rays).
When a heart or the vicinity of a heart is imaged using the X-ray CT apparatus, the reconstructed image includes artifacts caused by the heart beat. Several techniques to eliminate or suppress the artifacts are known. For example, Japanese Patent Laid-Open No. 2002-11001 discloses an ECG-gated reconstruction technique such as helical half-scan in which data for 180°+α (α is a fan angle) for a certain period of a heart beat are extracted from data acquired during the scan, e.g., three rotations in which a detector passes through a target slice position, and an image is reconstructed based on the extracted data.
With another ECG-gated reconstruction technique, projection data are acquired after a certain time from an R-wave in an electrocardiogram, and the data are collected in a period in which a heart has almost the same size until projection data with different projection directions are acquired, e.g. for 360° of a rotation angle. A cross-sectional image is reconstructed based on the projection data. This cross-sectional image is free from artifacts caused by variation in heart size. This technique is disclosed in Japanese Patent Laid-Open No. 2000-51208.
Unfortunately, the ECG-gated reconstruction requires a very long scanning time. When a cross-sectional image in a contraction period is obtained using an apparatus with 750 msec/rotation, the duration of time that is four times of a heart beat period is necessary to obtain projection data of the contraction period for 360° because a contraction period is typically 200 msec. Since the heart beat period is typically one second, a scanning time becomes as long as four seconds.
Another technique uses data of a specific phase of a heart to reconstruct an image during measurement of an EKG signal in an electrocardiogram. The phase of a heart is preferably as specific as possible and a range for the heart phase is empirically determined by using a standard value from technical literature. Furthermore, an ECG-triggered CT exposure technique disclosed in Japanese Patent Laid-Open No. 2000-157535 uses empirically defined delay time relative to R-wave. With this technique, data of a specific phase of a heart are used for image reconstruction.
Recently, a CT X-ray apparatus utilizing a cone beam (cone beam CT apparatus) has been developed. Typically, a regular X-ray CT apparatus emits an X-ray beam diverging in X-axis and Y-axis directions, which is called a fan beam. The cone beam CT apparatus emits an X-ray beam diverging in X-axis, Y-axis, and Z-axis directions, which is called a cone beam.
The cone beam CT apparatus suffers from not only artifacts due to the heart beat of a subject but also artifacts due to a big cone angle. When the entire chest is imaged in one rotation or half rotation using a flat panel detector (FPD) with a size of 43×43 cm and a cone angle of 10° or less, a focus to detector distance (FDD) needs to be about 2.5 m. When the FDD is 2.5 m, an X-ray generator cannot be rotated and so a human body needs to be rotated instead.
A typical X-ray room in a hospital has a width of 5 m, a length of 5 m, and a height of 4 m. The fastest speed to rotate a human body for one rotation is 3–5 seconds. The above-described segmental reconstruction data can be applied only to a high-speed CT apparatus, which takes only 0.4 seconds for one rotation. However, with a CT apparatus that rotates a human body, the segmental reconstruction cannot be performed, resulting in occurrence of artifacts due to the heart beat.
The beat motion can be divided into two phases, a contraction period and an isovolumic relaxation period. Since the contraction period has a large momentum, an image reconstructed using projection data in the contraction period tends to induce artifacts. Thus, when a lung field is diagnosed instead of a heart, preferably the projection data in the isovolumic relaxation period are used for image reconstruction. It is empirically known that the contraction period accounts for about 30% to 40% of one heart beat period.
In another ECT-gated imaging, an image is reconstructed using data from the isovolumic relaxation period that are selected from data acquired through a number of rotations of the apparatus. Since known CT apparatuses have a large fan angle, data from a number of rotations are necessary. Unfortunately, a human body cannot be rotated many times because a subject may feel dizzy from the rotations. Thus, preferably the subject is rotated only once.